Fix E0208: Invalid Constraint in Const Generic Default

Mon, 20 Jul 2026 12:44:04 +0800

1. Symptoms

When you encounter error E0208, the Rust compiler produces output similar to the following:

error[E0208]: constraints in const generic parameters must be satisfied by every concrete value
  --> src/main.rs:5:12
   |
5  | struct Wrapper<const N: usize = K, const K: usize = 4>();
   |                     ^^^^^^^^

The compiler is telling you that the constraint specified on a const generic parameter cannot be verified against the default value provided for that parameter. This typically manifests when you attempt to use a const generic parameter in the constraint of another const generic parameter with a default.

Constraints on ErrorVault — Developer Error Code Dictionary

Fix E0208: Invalid Constraint in Const Generic Default

1. Symptoms